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Exiting nerve root of spinal nerve S2

The exiting nerve root of spinal nerve S2 is the mixed spinal nerve that leaves the spinal canal through the second sacral foramen. It is formed by the fusion of the dorsal and ventral roots of S2 after they unite within the thecal sac. The nerve root passes laterally, accompanied by vessels, to join with other sacral nerves and contribute to the sacral plexus.

The S2 exiting root is clinically relevant because of its role in sensory and motor innervation of the posterior thigh, leg, foot, perineum, and its parasympathetic contributions to bladder, bowel, and sexual function.

Synonyms

  • Sacral spinal nerve root S2 (exiting)

  • Second sacral nerve (exiting component)

  • Exiting sacral root (S2)

Origin, Course, and Exit

  • Origin:

    • Formed by the union of dorsal and ventral roots of S2 in the thecal sac

  • Course:

    • Passes laterally from the dural sac through the intervertebral and sacral foramina

    • Travels with accompanying segmental vessels

    • Joins other sacral spinal nerves to participate in formation of sacral plexus branches

  • Exit:

    • Exits the spinal canal via the second sacral anterior and posterior sacral foramina

Relations

  • Anteriorly: Sacral ala, pelvic viscera

  • Posteriorly: Sacral lamina and posterior sacral foramina

  • Superiorly: Exiting S1 spinal nerve

  • Inferiorly: Exiting S3 spinal nerve

  • Laterally: Sacral plexus contributions (sciatic, pudendal, gluteal nerves)

Function

  • Motor contributions:

    • Supplies muscles of the posterior thigh, leg, and foot via sacral plexus branches

    • Contributes to pelvic floor and perineal muscle innervation

  • Sensory contributions:

    • Provides sensation to posterior thigh, leg, foot, and perineum (via posterior femoral cutaneous and pudendal nerves)

  • Autonomic contributions:

    • Parasympathetic fibers involved in bladder, bowel, and sexual function

Clinical Significance

  • Radiculopathy: Compression at the sacral foramen can cause pain radiating to posterior thigh, calf, foot, and perineum

  • Pelvic dysfunction: Lesions may affect bladder and sexual function

  • Surgical importance: At risk in sacral surgery, pelvic resections, and sacroiliac fusion procedures

  • Imaging: Key landmark in evaluating sacral plexopathy, tumors, and foraminal lesions

MRI Appearance

T1-weighted images:

  • Exiting nerve root shows low-to-intermediate signal intensity

  • Surrounded by bright epidural and perineural fat within the sacral foramen

T2-weighted images:

  • Appears as intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF at its origin and fat in the foramen

  • Pathological changes appear as more hyperintense or thickened nerve root

T1 Fat-Sat Post-Contrast:

  • Normal nerve root shows little or no enhancement

  • Pathological root (neuritis, tumor infiltration, metastasis) shows focal or diffuse enhancement

3D T2 SPACE / CISS:

  • Exiting nerve root shows intermediate to mildly hyperintense signal compared to muscle

  • Clearly outlined by bright CSF at its root origin and fat in the foramen

  • Excellent for detecting nerve compression, displacement, or infiltration within sacral foramina

MRI images

Exiting nerve root of spinal nerve S2 MRI coronal anatomy image-img-00000-00000